1) Investigate the Requirements for Subnetting

Next by studying network diagrams etc. decide the total number of subnets, keep in mind that VLANs also require separate subnets to operate.

Decide the number of IP addresses each subnet will require, this will also include IP addresses for the router interfaces and switch IP addresses which are to be used for telnet, ssh etc.

Decide to use a single subnet mask for all subnets or variable length subnet mask (VLSM) to avoid wastage of IP addresses.

2) Design your Subnets to Meet your Requirements

At design stage you will:

Select the Internet Addressing Architecture

Select the Subnet Mask

Make list of all subnets to use

Internet Addressing Architecture

Internet Addressing Architecture divides the IP addresses into to two address spaces.

Private IP Addresses: Packets addresses with private IP address cannot be routed or transmitted through the Internet, however; they can be used by enterprises within their local networks. Following is the list of private IP addresses:

Public IP Addresses: The Internet design requires that any one connecting to the Internet has to use a Public IP address, public IP addresses can only be routed through Internet. The problem face the IPv4 public IP address space is that it exhausted in 2011, to solve the issue the Internet community come-up with solutions like:

128-bit IPv6

Network Address Translation (NAT); which allows the mapping of hundreds of private IP addresses to a single or more than one public IP address

Select the Subnet Mask

Once you decided on the number of subnets, host per subnet required, type of subnet and choose the Internet addressing architecture, next step would be to select the mask. Subnet mask define the network, subnet and host part of the network.

Mask selected must have enough subnet bits to number all subnets and at the same time the host part bits must also be large enough to accommodate the number of hosts in each subnet.

It’s worth mentioning here:

That the difference between Classful and Classless Networks is that Classful network use the default mask define for the Class (e.g. Class A’s default mask is 255.0.0.0), while the Classless network use the variable length subnet mask.

Subnet mask can be represented in decimal (255.0.0.0), binary (11111111.00000000.00000000.00000000), or prefix (/8)

Make List of All Subnets to Use

Find the subnet ID, subnet broadcast for each subnet, and IP addresses to use.

Subnet ID is the numerically the smallest number in subnet and it can’t be used as IP address for host.

Subnet Broadcast address is the numerically highest number in subnet and it is used for directed broadcast in the subnet.

IP addresses are the addresses between the subnet ID and subnet broadcast address, it can be used as host’s IP address.

3) Decide How to Implement the Subnets

Base on your network diagram identify the location for subnet, pick one from the list. Separate the static IP addresses and dynamic IP addresses assigned through DHCP.